JP6177915B2 - Scanning electron microscope - Google Patents

Description

  The present invention relates to a scanning electron microscope that detects signal electrons that have passed through an objective lens.

By detecting the signal electrons generated when scanning the sample with the irradiation electron beam focused on the sample, and displaying the signal intensity at each irradiation position in synchronization with the scanning signal of the irradiation electron beam, scanning electron microscope to obtain a two-dimensional image of the scanned area (S canning E lectron M icroscope: SEM) is widely known.

  In general SEM, chromatic aberration increases in a low acceleration range, and high resolution cannot be obtained. In order to reduce this chromatic aberration, a deceleration method is effective in which the objective lens is passed at a high speed and the irradiated electron beam is irradiated while being decelerated immediately before the sample. By applying the decelerating method, the influence of charging and damage associated with electron beam irradiation is reduced, and information on the surface of the sample electrode can be obtained with high resolution. For this reason, low-acceleration SEM is used for observation of sample surfaces in a wide range of fields.

On the other hand, signal electrons detected by SEM are roughly classified into backscattered electrons and secondary electrons depending on the energy emitted from the sample surface. Irradiating electron beam irradiation undergoes an elastic or inelastic scattering in the sample, which was released to the outside of sample backscattered electrons: called (B ack s cattered E lectron BSE ), inelastic scattering process of backscattered electrons in generating a low energy of the signal electrons secondary electrons emitted from the sample surface to the outside of the sample (S econdary E lectron: SE) and called. An example of the energy distribution of secondary electrons (SE) and backscattered electrons (BSE) generated when the energy of the irradiated electron beam is E0 is shown in FIG. In general, a signal electron with an energy of less than 50 eV is called SE, and has a peak of generation amount with an energy of several eV. On the other hand, BSE has a peak at the same energy level as irradiated electrons. The amount of secondary electrons and backscattered electrons generated depends on the elements constituting the sample and the energy of the irradiated electron beam, but generally the amount of secondary electrons generated is larger than that of backscattered electrons.

  The amount of backscattered electrons generated depends on the average atomic number, density, and crystallinity of the sample at the irradiation electron beam irradiation position. In an SEM image in which only backscattered electrons are detected without detecting secondary electrons, a contrast reflecting differences in the composition and crystal orientation of the sample can be obtained. On the other hand, since secondary electrons are generated on the sample surface, a contrast reflecting the unevenness of the sample and the difference in potential can be obtained. Thus, different sample information can be obtained by separately detecting secondary electrons and backscattered electrons. For this reason, many SEMs equipped with a plurality of detectors are often seen for the purpose of separately acquiring various sample information obtained by SEM observation.

In particular, in SEM observation in a low acceleration region, in order to obtain high resolution, it is necessary to reduce the aberration that occurs when the irradiation electron beam passes through the objective lens. Therefore, the distance (working distance (W orking D istance: WD) ) between the sample and the objective lens tip has to be shorter than a few mm to. When the decelerating method is applied under such observation conditions, most of the signal electrons pass through the objective lens while being accelerated. Therefore, the detector for detecting the signal electrons may be installed closer to the electron source than the objective lens. In an SEM equipped with such a detection system, there is a tendency that importance is attached to a function capable of acquiring different sample information separately for each detector.

JP2012-15130 JP2000-299078 International Publication No. 01/075929

  As a result of intensive studies on low-acceleration observation of an SEM to which the deceleration method is applied, the present inventor has obtained the following knowledge.

Compared with transmission electron microscopes and scanning transmission electron microscopes, SEM continuously provides a wide magnification range from several hundred times to several hundred thousand times by changing the scanning range of the irradiated electron beam even when the sample size exceeds several mm ³ There is an advantage that can be observed by changing. However, in the case of a method in which the signal electrons that have passed through the objective lens are detected by a detector installed between the electron source and the objective lens, the signal electrons reach the detector via a lens field or deflection field on the orbit. Therefore, the arrival position of the signal electrons may depend on the generation position on the sample. For this reason, part of the SEM image has nothing to do with the sample shape due to the fact that the signal electrons do not reach the detector sensitive surface, especially when the signal electrons are largely deflected off-axis and observed at a low magnification. A shadow-like contrast may appear. If such a phenomenon is called shading, it is desirable to avoid shading as much as possible when observing an SEM image.

  On the other hand, in the SEM, different information is included in secondary electrons and backscattered electrons detected as signal electrons. It is known that when detecting secondary electrons generated on the sample surface, information on unevenness and potential can be obtained, and when backscattered electrons are detected, an image in which information on composition and crystal orientation is emphasized can be obtained. Therefore, if there is a mechanism that can separately detect the secondary electrons and backscattered electrons generated at the same irradiation electron beam irradiation position, different information can be acquired simultaneously in the same observation field.

  In particular, when performing elemental analysis and crystal orientation analysis such as EDX (Energy Dispersive X-ray Spectroscopy) and EBSD (Electron Backscatter Diffraction), backscattering is required. In many cases, an electron image is used, and an SEM that can obtain a contrast derived from backscattered electrons regardless of the magnification is required. However, the amount of backscattered electrons is less than that of secondary electrons, and depending on the detection rate, it may be difficult to perform focus adjustment while observing an SEM image in which only backscattered electrons are detected. For this reason, it is convenient if there is a configuration in which the same observation field of view can be adjusted with a SEM image of a large amount of secondary electrons to obtain a SEM image of backscattered electrons. Furthermore, when acquiring different sample information in the same field of view, if it is possible to observe without shading from high magnification observation to low magnification observation, desired sample information can be easily obtained, which is convenient.

  In Patent Document 1, among two detectors installed in a cylindrical electrode having a higher potential than a sample, secondary electrons are shielded by a detector installed on the electron source side and equipped with an energy filter. A means for detecting backscattered electrons and detecting secondary electrons with a detector installed on the sample side is disclosed. When the detection system is configured in this way, it is necessary to provide a passage hole at the center of the sample side detector in order to detect backscattered electrons with the electron source side detector. Shading occurs when a low-magnification SEM image is acquired with the sample-side detector alone. This is because when the irradiation electron beam is deflected, the signal electrons generated off the axis travel off the axis under the influence of the lens field and the deflection field after passing through the objective lens. The signal electrons generated on the axis are not detected when the signal electrons flying near the optical axis pass through the passage hole of the sample side detector, while the signal electrons generated off the axis are detected by the sample side detector. The signal electrons that should be detected by the electron source side detector are also detected by the sample side detector. For this reason, when an SEM image is acquired with a detector alone installed on the sample side for detecting secondary electrons, the center of the SEM image is dark regardless of the sample information, and the outer side has a bright contrast, i.e., shading. Observed. As described above, the detector arrangement described in Patent Document 1 has a configuration in which a secondary electron image and a backscattered electron image are separately obtained. However, the detector of either the sample side detector or the electron source detector can be used. In principle, the acquired SEM image cannot avoid shading that occurs during low-magnification observation.

  Further, the backscattered electrons detected by the electron source side detector are limited to those emitted in the vicinity of the optical axis direction. When the sample is flat, the angular distribution of generated signal electrons is known to follow the cosine law, and the number of backscattered electrons generated in the optical axis direction is small. For this reason, when backscattered electrons are detected by this method, the backscattered electrons to be detected are about several percent of the total amount generated, and are vulnerable to irradiation damage of irradiated electron beams such as biological samples, and the probe current is sufficient. When observing a sample that cannot be set large, an SEM image with sufficient contrast cannot be obtained.

  Patent Document 2 discloses a technique for separately detecting secondary electrons and backscattered electrons that have passed through the objective lens field in a SEM equipped with an objective lens on which an electromagnetic field is superimposed by applying a deceleration method. Has been. Here, the secondary electron detector is installed on the electron source side, and the backscattered electron detector is installed on the sample side. In this configuration, the backscattered electron detector does not include a secondary electron shielding means such as an energy filter, and the signal electrons detected by each detector are controlled by controlling the electromagnetic field formed on the trajectory of the signal electrons. It is a sort of energy. For this reason, when the orbit of secondary electrons and backscattered electrons is changed by changing the acceleration voltage or WD, it is not easy to separate and detect desired sample information. Although Patent Document 2 does not mention shading, when a deflector is installed at the position shown in the drawing, secondary electrons generated off-axis are transferred to the sample-side detector when the probe electrons are largely deflected. This is considered to be shielded, and in the low-magnification observation of the SEM image by this detector, it is considered that shading occurs where the outside of the field of view becomes dark regardless of the sample information.

  Patent Document 3 discloses a means for separating and detecting secondary electrons and backscattered electrons that have passed through the objective lens field in an SEM equipped with an objective lens on which an electromagnetic field is superimposed by applying a deceleration method. Similarly to Patent Document 1, the backscattered electron detector is installed on the electron source side, the secondary electron detector is installed on the sample side, and an energy filter is provided in front of the backscattered electron detector. Without deflecting the irradiated electron beam off-axis, the low-energy signal electrons generated when the backscattered electrons that have passed through the energy filter collide with the conversion electrode are guided to the detector sensing surface installed off-axis. The electrons are configured to detect low energy conversion electrons generated when colliding with the energy filter. In this method, two identical Wien filters that apply an electric field and a magnetic field in directions perpendicular to the optical axis and perpendicular to each other are mounted. With this configuration, the electric fields and magnetic fields of the two Wien filters are applied in opposite directions, and when the crossover of the irradiated electron beam is set at the midpoint between the two Wien filters, the chromatic dispersion generated in each Wien filter can be canceled. In general-purpose equipment, it is necessary to configure the equipment so that observation is always possible under optimum conditions when the acceleration voltage and WD are changed. For this reason, when the probe current and the opening angle on the sample are controlled independently with a general-purpose scanning electron microscope, a configuration in which two condenser lenses are mounted is common, but in order to realize the method of Patent Document 3 as a general-purpose device. In order to always control the crossover position to the middle point of the Wien filter, it is necessary to mount another condenser lens. Such a configuration is not preferable as a configuration of a general-purpose device because optical axis adjustment and optical system control are complicated.

  In this method, the secondary electrons are shielded by an energy filter, and the backscattered electrons that have passed are detected. Because low-energy conversion electrons generated when backscattered electrons collide are detected by a detector installed off-axis, conversion electrons generated on the detector side are more easily detected than the optical axis, and are detected from the optical axis. There is a possibility that shading resulting from the difficulty of detecting the converted electrons generated on the opposite side of the device will be observed in a low-magnification SEM image.

  In addition, it is known that the generation efficiency of conversion electrons is maximized when it collides with, for example, a metal conversion electrode at an energy of about 1 keV, and the conversion efficiency is higher when electrons with higher energy collide. Decreases. For this reason, when the irradiation energy of the irradiated electron beam to the sample is E0, the energy range of the generated backscattered electrons is E0 or less. When E0 ≧ 1 keV, the detection efficiency of signal electrons near an energy of 1 keV is high. However, when E0 >> 1 keV, the conversion efficiency is low and the detection efficiency is low.

  The purpose of the present invention is to detect a signal electron that has passed through the objective lens in low-acceleration observation of an SEM using the deceleration method, and a wide magnification range from a low magnification of about several hundred times to a high magnification of 100,000 times or more. The present invention relates to providing an SEM that can be acquired without the influence of shading, and to provide a SEM that can detect backscattered electrons that are generated less than secondary electrons with high efficiency.

  The present invention provides, for example, two detection means having a sensitive surface with an axially symmetrical arrangement with respect to the optical axis for detecting signal electrons accelerated by passing through the objective lens with an SEM to which a decelerating optical system is applied. And a first detector for detecting backscattered electrons that have passed without being shielded by an energy filter installed in front of the sensing surface in order to shield the secondary electrons, A second detector for detecting backscattered electrons is provided, the first detector is closer to the sample than the second detector, and the output signals from the first detector and the second detector are linearly added. L1 / L2 when the distance between the sensitive surface of the first detector and the tip of the objective lens is L1, and the distance between the sensitive surface of the second detector and the tip of the objective lens is L2. It relates to the arrangement satisfying ≦ 5/9.

  According to the present invention, for example, a secondary electron image and a backscattered electron image can be acquired simultaneously and separately while observing the surface of the sample electrode with high resolution in a low acceleration region of 5 kV or less. At this time, by arranging the detectors as described above, the solid angle of the through hole provided at the center of the second detector facing from the tip of the objective lens is reduced, and the second detector in the high magnification observation is reduced. Shading caused by passing through the center can be reduced. In addition, signal electrons that fly off-axis in low-magnification observation and are not detected by the second detector can be detected by the first detector. For this reason, shading in low-magnification observation can be reduced by outputting linear addition signals of the first detector and the second detector. As described above, it is possible to provide an SEM in which the influence of shading is reduced over a wide magnification range.

  Further, according to the present invention, for example, the first detector for detecting backscattered electrons having a sensitive surface shape which is axisymmetric with respect to the optical axis is a second detector for detecting secondary electrons. Rather than the sample side. For this reason, when the angle distribution of the signal electrons generated when a flat sample is assumed is taken into consideration, the first detector can detect an angular band with a larger generation amount than the second detector, and therefore, the backscattering can be performed more than before. An SEM that can detect electrons efficiently can be provided.

It is a figure which shows the energy distribution figure of a common emitted electron. 1 is a schematic cross-sectional view showing a scanning electron microscope according to Example 1. FIG. 1 is a schematic cross-sectional view showing a scanning electron microscope according to Example 1. FIG. 1 is a schematic cross-sectional view showing a scanning electron microscope according to Example 1. FIG. 1 is a schematic cross-sectional view showing a scanning electron microscope according to Example 1. FIG. 6 is a schematic cross-sectional view showing a scanning electron microscope according to Example 2. FIG. 6 is a schematic cross-sectional view showing a scanning electron microscope according to Example 2. FIG. 6 is a schematic cross-sectional view showing a scanning electron microscope according to Example 2. FIG. 6 is a schematic cross-sectional view showing a scanning electron microscope according to Example 2. FIG. 6 is a schematic cross-sectional view showing a scanning electron microscope according to Example 2. FIG. 6 is a schematic cross-sectional view showing a scanning electron microscope according to Example 2. FIG.

  Examples include an electron source for generating an electron beam to be a probe, an aperture for limiting the diameter of the electron beam, a sample stage on which a sample to be irradiated with the electron beam is mounted, and an object for focusing the electron beam on the sample surface. An electron lens including a lens, a deceleration unit that decelerates as the electron beam passes through the objective lens as it approaches the sample, a deflector that scans the electron beam on the sample, and signal electrons emitted from the sample, It has at least two detectors for detecting signal electrons that have passed through the objective lens, the two detectors are arranged between the electron source and the objective lens, and the two sensitive surfaces have an axisymmetric shape with respect to the optical axis. One of the detectors is a detector arranged exclusively to detect high-energy signal electrons that have passed through the deceleration electric field type energy filter, and the first detector of the detectors is the first detector. Different, another When the detector is the second detector, the first detector is installed on the sample side of the second detector, and the distance between the sample side tip of the objective lens and the sensitive surface of the first detector is Disclosed is a scanning electron microscope in which L1 / L2 ≦ 5/9, where L1 is the distance between the tip of the objective lens on the sample side and the sensitive surface of the second detector.

  In addition, the embodiment discloses that a signal processing circuit for linearly adding the output signals from the first detector and the second detector is provided.

  In addition, the embodiment discloses that the first detector detects backscattered electrons and the second detector detects secondary electrons.

  In addition, the embodiment discloses that the deceleration electric field type energy filter is provided as a unit independent of the first detector.

  In addition, the embodiment discloses that the deceleration electric field type energy filter is provided as a unit integrated with the first detector.

  In the embodiment, a deceleration electric field type energy filter is provided on the sample side from the sensing surface of the second detector, and the first detector and the second detector are separately passed through the deceleration electric field type energy filter. Detecting energetic electrons is disclosed.

  In addition, the embodiment discloses that the detection solid angle of the first detector facing from the tip of the objective lens is larger than the detection solid angle of the second detector.

  Further, in the embodiment, the detector used for the first detector or the second detector is a semiconductor detector, an avalanche diode, a micro channel plate, a detector using a scintillator material as a component, or a combination thereof. It is disclosed.

  In addition, the embodiment includes an electron source that generates an electron beam to be a probe, an aperture that limits the diameter of the electron beam, a sample stage on which a sample to be irradiated with the electron beam is mounted, and the electron beam is focused on the sample surface An electron lens including an objective lens, a decelerating means for decelerating as the electron beam passes through the objective lens, a deflector that scans the electron beam on the sample, and a signal electron emitted from the sample Among them, at least two conversion plates that collide with signal electrons that have passed through the objective lens are provided, the two conversion plates are arranged between the electron source and the objective lens, and the collision surfaces of the two conversion plates are aligned with respect to the optical axis. One of the conversion plates has a symmetric shape, and the first conversion plate and the first conversion plate are arranged such that high-energy signal electrons that have passed through the deceleration electric field type energy filter collide exclusively. Different from 1 conversion board When the other conversion plate is the second conversion plate, the first conversion plate is placed closer to the sample side than the second conversion plate, and the front end of the objective lens on the sample side and the collision surface of the first conversion plate A scanning electron microscope in which L1 / L2 ≦ 5/9 is disclosed, where L1 is a distance between and L2 is a distance between the tip of the objective lens on the sample side and the collision surface of the second conversion plate.

  In addition, the embodiment includes a sensitive surface for detecting conversion electrons emitted from the collision surface to the sample side by the signal electrons colliding with the first conversion plate, and is arranged symmetrically with respect to the optical axis outside the optical axis. And a sensing surface for detecting conversion electrons emitted from the collision surface to the sample side by the signal electrons colliding with the second conversion plate. It is disclosed that the third and fourth detectors are arranged symmetrically with respect to the axis. Also disclosed is a signal processing circuit for linearly adding the output signals from the first, second, third, and fourth detectors.

  In addition, the embodiment discloses detecting conversion electrons generated by collision of backscattered electrons with the first conversion plate and detecting conversion electrons generated by collision of secondary electrons with the second conversion plate. .

  In addition, the embodiment discloses that the deceleration electric field type energy filter is provided as a unit independent of the first conversion plate.

  Further, the embodiment discloses that the deceleration electric field type energy filter is provided as a unit integrated with the first conversion plate.

  Further, in the embodiment, a deceleration electric field type energy filter is provided on the sample side from the collision surface of the second conversion plate, and each of the first conversion plate and the second conversion plate separately passes through the deceleration electric field type energy filter. Disclose that energy electrons collide.

  Further, the embodiment discloses that the collision solid angle of the first conversion plate facing from the tip of the objective lens is larger than the collision solid angle of the second conversion plate.

  Also, the examples disclose that the detectors used in the first, second, third, and fourth detectors are detectors that use scintillator material as a component or a combination thereof.

  In addition, the embodiment discloses that a material having an atomic number of 50 or more is included in the collision surfaces of the first and second conversion plates.

  Moreover, it discloses that the material which has a negative electron affinity is contained in the collision surface of a 1st, 2nd conversion board in an Example.

  Hereinafter, the above and other novel features and effects of the present invention will be described with reference to the drawings. The drawings are used exclusively for understanding the invention and do not limit the scope of rights.

  FIG. 2 is a conceptual diagram of the overall configuration of the scanning electron microscope according to the present embodiment. The scanning electron microscope shown in FIG. 2 roughly includes an electron gun 4 having a mechanism for irradiating a sample 15 with an irradiation electron beam, an aperture for limiting the diameter of the irradiation electron beam, a condenser lens, and an objective lens. An electron lens such as a detector for detecting secondary electrons 2, a detector for detecting backscattered electrons 3, an energy filter 9 A for shielding secondary electrons 2, a deflector, and a sample 15. It has a sample stage 16 and its mechanism, a display device for SEM images, a controller for controlling the entire SEM, and an evacuation facility for locating and observing the observation area. The arrival position of each signal electron on the detector sensing surface depends on the deflection field formed on the signal electron trajectory, but the signal electron trajectory accelerated by the deceleration optical system is essentially the deflection field. You can think of it as position independent. For this reason, the deflector installation position is arbitrary in this embodiment.

  In FIG. 2, the sensitive surface positions of the detector for detecting secondary electrons and backscattered electrons are shown as the sensitive surface 7 of the second detector and the sensitive surface 8 of the first detector, respectively. As shown in FIG. 2, the sensitive surface 8 of the first detector for detecting backscattered electrons is installed closer to the sample 15 than the sensitive surface 7 of the second detector for detecting secondary electrons. .

  The electron gun 4 corresponds to any of various electron guns such as CFE (Cold Field Emission), SE (Schottky Emission), and thermionic emission. The electron gun mounted on the scanning electron microscope is selected from these according to the desired apparatus performance.

  The objective lens of the scanning electron microscope shown in this embodiment is an out-lens type with a small leakage magnetic field with respect to the sample 15, and the cylindrical electrode 10 is installed along the inner wall of the magnetic path of the objective lens of the scanning electron microscope. The cylindrical electrode 10 is set at a higher potential than the objective lens magnetic path 12. Thereby, a decelerating electric field for the irradiation electron beam is formed between the sample side end 12 of the cylindrical electrode and the sample side end 13 of the objective lens magnetic path, and gradually decelerated when the irradiation electron beam passes. It has become. The potential difference between the objective lens magnetic path 12 and the sample 15 is set within 1 kV. The cylindrical electrode 10 and the objective lens magnetic path 12, and the gap between the cylindrical electrode 10 and the SEM barrel 6 are configured to be electrically insulated by an insulator (not shown).

  In order to obtain the same effect, the cylindrical electrode 10 made of a magnetic material can be used as part of the objective lens magnetic path 12 as shown in FIG. In this case, the cylindrical electrode 10 and the objective lens 12 are configured to be electrically insulated from each other by an insulator (not shown) as long as they are magnetically coupled.

  In particular, in order to obtain high resolution under an observation condition where the irradiation voltage of the irradiation electron beam is 5 kV or less, the cylindrical electrode 10 needs to be at a high potential with respect to the sample 15 in order to form a deceleration electric field. This potential difference is set to Vd. In the case of the configuration of this embodiment, Vd is set to about 10 kV. Furthermore, in order to reduce chromatic aberration that occurs when the objective lens passes, the distance (WD) between the sample 15 and the sample side end portion 13 (tip portion) of the objective lens magnetic path is set to 10 mm or less. It is desirable.

  When the deceleration electric field is configured in this way, the electric field lens action formed is the same if the potential difference Vd is the same. For this reason, the method called the acceleration-deceleration method in which the sample is grounded and the light source side is higher than that is the same as the method called the deceleration method in which the objective lens is grounded and the sample is a negative potential. If the potential distribution is the same, the same electric field lens action can be obtained. For this reason, hereinafter, the acceleration-deceleration method and the deceleration method will be unified and described as the deceleration method without distinguishing between them.

  Under the observation conditions as described above, a part of the signal electrons 1 generated in the sample 15 is caused by the electric field formed between the sample side end 11 of the cylindrical electrode 10 and the objective lens magnetic path end 13 and the objective lens 12. It is converged by the formed magnetic field, accelerated by the electric field, and proceeds in the direction opposite to the irradiation electron beam.

  The sensitive surface 8 of the first detector and the sensitive surface 7 of the second detector are both arranged symmetrically with respect to the optical axis and have the same potential as the cylindrical electrode 10. As a result, there is a potential difference Vd between the sample 15 and the sensitive surface 8 of the first detector and the sensitive surface 7 of the second detector, and some of the signal electrons generated in the sample 15 are generated by the first detector. It reaches the sensing surface 8 or the sensing surface 7 of the second detector with an acceleration of about 10 keV. This is sufficient energy for detection with an existing electron detector.

  As the first detector and the second detector, those which can achieve the arrangement of the sensing surfaces shown in the figure and can detect the signal electrons accelerated by the potential difference Vd are used. A semiconductor detector, an avalanche diode, a micro channel plate, a detector using a scintillator material as a constituent element, and the like can be considered, and any type of detector may be used. If there is no problem in linear addition of signals described later, different types of detectors may be used for the first detector and the second detector.

  In many cases, in order to detect signal electrons, a detector using a scintillator material is typically installed on the sensitive surface from the viewpoint of multiplication factor and responsiveness. When this is used as the first detector and the second detector of this embodiment, signal electrons are detected on the same principle as an Everhart & Thornley type (hereinafter referred to as ET type) detector generally used as a detector of a scanning electron microscope. it can. This detector includes a scintillator that converts accelerated signal electrons into light, and a photomultiplier tube that converts light into photoelectrons and amplifies the photoelectrons, and the scintillator and photomultiplier tubes are connected by a light guide. The The scintillator emits sufficient light if the incident signal electrons have an energy higher than 5 keV or more, and guides this light to the sensitive surface of the photomultiplier tube via the light guide. Can be detected as an electrical signal. Since the scintillator is an insulator, charging occurs when signal electrons collide, and in some cases, the scintillator may be decelerated immediately before reaching the sensitive surface. In order to avoid this, it is desirable to coat the surface of the scintillator by depositing a conductor such as Al. This metal coating also has an effect of reflecting light generated by the scintillator to the photomultiplier tube side without leaking to the outside. The sensitive surface 8 of the first detector and the sensitive surface 7 of the second detector shown in FIG. 2 are synonymous with the conductor surface that covers the scintillator surface. In this case, the conductor on the scintillator surface and the cylindrical electrode 10 have the same potential.

  Note that a method may be used in which low-energy conversion electrons generated when accelerated high-energy electrons collide with the conversion electrode are detected by an ET type detector installed outside the optical axis. In this case, the conversion electrode is regarded as the sensitive surface of the first detector or the second detector, and the conversion electrode has the same potential as the cylindrical electrode 10. The same potential as that of the cylindrical electrode 10 or a potential larger than that is set on the sensitive surface of the detector installed off-axis. Thereby, the low energy conversion electrons generated on the conversion electrode are collected on the sensitive surface of the off-axis detector. In the case of a method in which the conversion electrode is detected by an off-axis detector, the number of converted electrons reaching the detector sensing surface changes depending on the distance between the conversion electron generation site and the off-axis detector, which is the cause of shading. There is a possibility. Therefore, as shown in FIG. 4, the first detectors 18A and 18B for detecting the converted electrons off-axis and the second detectors 17A and 17B for detecting the converted electrons are arranged symmetrically with respect to the optical axis. By installing at, shading can be avoided in low magnification observation. In this case, it is expected that an SEM image with reduced shading can be obtained by linearly adding the output signals of two detectors arranged symmetrically with respect to the optical axis.

  The surface portion of the conversion electrode is preferably made of a material having a large amount of secondary electron emission. Typically, a metal film such as gold (Au, atomic number 79) is used. In order to obtain the same effect, for example, a film made of a material having a larger amount of secondary electron emission than a normal metal, such as magnesium oxide or diamond having a high electron affinity, may be used.

  When the first detector and the second detector are the same type of detector, backscattered electrons 3 and secondary electrons 2 flying on the detector sensing surface are detected. For this reason, when the backscattered electrons 3 are detected without detecting the secondary electrons 2 by the first detector, it is necessary to install an energy filter 9A in front of the sensitive surface 8 of the first detector.

  The energy filter 9A may be installed as a component integrated with the first detector, but may be installed as a component separate from the first detector. However, in the case of installation as a separate structure, all the signal electrons 1 that reach the sensitive surface 8 of the first detector always pass through the energy filter 9A before reaching the sensitive surface 8 of the first detector. Need to be placed in. A configuration that does not affect the trajectory of the irradiated electron beam due to a change in the electric field outside the energy filter accompanying the on / off filter is desirable.

  The trajectory of the signal electrons 1 depends on the electric field formed between the sample side end 11 of the cylindrical electrode and the sample side end 13 of the objective lens magnetic path and the magnetic field formed by the objective lens 12. For this reason, when the position of the sample 15 and the irradiation voltage of the irradiation electron beam are changed, the excitation necessary for focusing the irradiation electron beam on the surface of the sample 15 changes, and the trajectory of the signal electron 1 also changes accordingly. Since the electric field and magnetic field of the objective lens are controlled to focus the irradiation electron beam, the electric field and magnetic field cannot be controlled to control the trajectory of the signal electrons 1.

  In general SEM, various observation conditions such as acceleration voltage and WD are used. Therefore, the first detector and the second detector assume that the secondary electrons 2 and the backscattered electrons 3 are mixed and detected depending on the observation conditions. However, when obtaining an SEM image of the secondary electrons 2, the amount of secondary electrons 2 emitted according to the energy distribution of FIG. 1 is sufficiently larger than the backscattered electrons 3. Image quality close to an electronic image can be obtained. On the other hand, when it is desired to separate and detect the backscattered electrons 3 that are generated less than the secondary electrons 2, if the secondary electrons 2 are mixed, the information of the secondary electrons 2 is reflected in the SEM image, which is shielded. Therefore, an energy filter 9A is required.

  When the signal detected by the second detector is displayed as an SEM image in order to obtain the SEM image of the secondary electrons 2, the SEM image of the low magnification is displayed for the same reason as the electron source side detector of Patent Document 1. A shading that darkens is observed. In order to avoid this, in low magnification observation, the energy filter 9A is turned off, and the signal electrons 1 are detected by the first detector and the second detector. The influence of shading can be reduced by linearly adding the signals of both detectors and displaying them as an SEM image. In addition, since the signals detected by the two detectors are displayed as SEM images, the amount of signal increases compared to the case where the detection signal of the second detector alone is displayed as an SEM image, and the S / N is high. SEM images of secondary electrons are expected to be obtained.

  If the distance between the sensitive surface of the first detector and the sample side tip of the objective lens is L1, and the distance between the sensitive surface of the second detector and the sample side tip of the objective lens tip is L2, observation is performed by the above method. In order to reduce shading by low-magnification observation of the secondary electron image, and to detect backscattered electrons with high efficiency by the first detector, it is desirable to arrange so as to satisfy L1 / L2 ≦ 5/9 . The reason for this will be described below.

  Most of the signal electrons generated off-axis during low-magnification SEM observation progress away from the optical axis as they move away from the sample after passing through the objective lens. For this reason, signal electrons generated at a position where the off-axis distance from the optical axis is large are detected mainly by the first detector installed on the sample side. However, the area of the sensing surface of the detector cannot be set large without limitation. Therefore, in order to efficiently detect off-axis signal electrons with the first detector having a finite sensitive area, a configuration in which the sensitive surface is close to the sample, that is, a configuration with a small L1 is desirable.

  On the other hand, since the signal electrons generated near the optical axis are subjected to a convergence effect from the objective lens, the off-axis is relatively small. For this reason, most of the signal electrons generated near the optical axis pass through the central hole of the first detector and are detected by the second detector. As shown in FIG. 2, the first detector and the second detector are each provided with an electron passage hole in the center. For this reason, some of the signal electrons generated near the optical axis that should be detected by the second detector pass through the central hole of the second detector and are not detected. In order to effectively reduce the number of signal electrons passing through the center hole, a configuration in which the sensitive surface of the second detector installed on the electron source side is set at a position away from the sample, that is, a configuration with a large L2 is used. desirable.

  From the above, in terms of the configuration of the detector, a structure in which L1 / L2 is as small as possible is desirable, but the value of L1 / L2 is restricted by the arrangement of the objective lens and condenser lens that constitute the optical system. Since both the first detector and the second detector are installed between the objective lens and the electron source, the position of the sensitive surface of the first detector depends on the size and structure of the objective lens and the energy filter, and is unlimited. Cannot be installed near the sample. Further, when considering the arrangement of the components of the optical system for use by changing the acceleration voltage or WD, the position of the sensitive surface of the second detector cannot be placed at an unlimited distance from the sample. Considering the above constraints, the configuration of the electron optical system of the SEM is assumed, and as a result of examining the setting range of L1 and L2 from the trajectory of signal electrons, an arrangement satisfying L1 / L2 ≦ 5/9 is obtained. It was found that the influence of shading observed by the secondary electron detection method can be made smaller than that of the conventional method.

  Furthermore, when signal electrons are detected by the above method, backscattered electrons of 5 keV or less are converged in the process of passing through the objective lens, and the orbits of the signal electrons are widely dispersed due to the difference in energy. Therefore, it is expected that more backscattered electrons reach the sensitive surface of the first detector installed on the sample side than in the conventional method. Note that the smaller the center hole of the first detector, the larger the amount of backscattered electrons detected by the first detector, and the less the influence of shading observed in the SEM image acquired from the signal of the first detector. . Therefore, when mainly detecting backscattered electrons, the solid angle of the sensitive surface 8 of the first detector facing the sample side end 13 of the objective lens magnetic path is larger than the solid angle of the sensitive surface 7 of the second detector. A larger configuration is desirable.

  In order to reduce shading in the SEM image of backscattered electrons, a configuration is possible in which an energy filter 9B is provided separately from the first detector as shown in FIG. . A configuration is adopted in which all signal electrons 1 that reach the sensitive surface 7 of the second detector are arranged so as to pass through the energy filter 9B before reaching the sensitive surface 7 of the second detector. A SEM image of linearly added signals of the first detector and the second detector is displayed with both the energy filter 9A of the detector and the energy filter 9B of the second detector turned on, and an SEM image of backscattered electrons is obtained without shading. It becomes possible to do. In the SEM image in which the linear addition signals of the first detector and the second detector are displayed with both the energy filter 9A of the first detector and the energy filter 9B of the second detector turned off, the second order is used for the above reason. Electronic SEM images can be obtained without shading, and SEM images with a wide range of magnification can be provided for energy-selected SEM images. In addition, when acquiring the SEM image of backscattered electrons in the on state of the energy filter 9A of the first detector and the energy filter 9B of the second detector, it is desirable that the filter voltages can be set to different filter voltages.

  FIG. 6 is a conceptual diagram of the overall configuration of the scanning electron microscope of this embodiment. Hereinafter, the difference from the first embodiment will be mainly described.

  The scanning electron microscope shown in FIG. 6 roughly includes an electron gun 4, an aperture, a condenser lens, an objective lens, a second detector, a first detector, an energy filter 9A, a deflector, a sample 15, a sample stage 16, and its It has a mechanism, a SEM image display device, a controller that controls the entire SEM, and a vacuum exhaust system.

  The scanning electron microscope shown in FIG. 6 differs from Example 1 in the type of objective lens. The objective lens of this embodiment is a semi-in-lens type objective lens that intentionally leaks a magnetic field to the sample. Compared with the configuration of the first embodiment, higher resolution can be obtained.

  The principle that the influence of shading can be reduced with the SEM image obtained by adding the output signals of the first detector and the second detector is the same as in the first embodiment.

  In FIG. 6, the sample side end 13 of the objective lens magnetic path has a higher potential than the sample 15. This potential difference is set to Vd as in the first embodiment. In the case of the configuration of this embodiment, since a short focus is achieved by the magnetic lens, the electric field lens strength is weaker than that of the first embodiment, and Vd is set in the range of 1 to 5 kV. With this potential difference, when the detector is an ET type detector, the scintillator does not emit light. Therefore, the sensitive surface 7 of the first detector and the sensitive surface 8 of the second detector are more independent of the objective lens 12 than the sample 15. It is desirable to set a high potential of about 10 kV. In addition, when detecting the conversion electrons generated by colliding with the conversion electrode described in the first embodiment, the sensitive surface 7 of the first detector and the sensitive surface 8 of the second detector have the same potential as the objective lens magnetic path 12. There is no hindrance.

  When the sample is tilted in the configuration of FIG. 5, the symmetry of the electric field between the sample side end 13 of the objective lens magnetic path and the sample 15 is lost, and the resolution significantly decreases. In order to avoid this, as shown in FIG. 7, an electrode 14 for controlling the electric field may be separately provided on the sample side of the lower magnetic path of the objective lens 12. In this case, the electrode 14 needs to be a nonmagnetic material. As in the first embodiment, the potential difference between the electrode 14 and the sample 15 is preferably set within 1 kV.

  Furthermore, as in the first embodiment, as shown in FIG. 8, a cylindrical electrode 10 that includes the first detector and the second detector may be provided. In this configuration, the potential difference between the sample-side end 11 of the cylindrical electrode and the electric field control electrode 14 is configured to be Vd.

  In order to increase the yield of the signal electrons 1 detected by the first detector and the second detector, as shown in FIG. 9, the upper magnetic path of the objective lens and the cylindrical electrode 10 may be combined. In this case, the cylindrical electrode 10 is made of a magnetic material.

  The configuration described above is expected to obtain the same effect even when applied to the monopolar objective lens shown in FIG. 10 or the in-lens objective lens shown in FIG.

DESCRIPTION OF SYMBOLS 1 ... Signal electron 2 ... Secondary electron 3 ... Backscattered electron 4 ... Electron gun 5 ... Optical axis 6 ... SEM barrel 7 ... Sensing surface 8 of 2nd detector ... Sensing surface 9A of 1st detector ... 1st detection Decelerated electric field type energy filter 9B for signal electrons 1 detected by the detector ... Decelerated electric field type energy filter 10 for signal electrons 1 detected by the second detector ... Cylindrical electrode 11 ... Sample side end 12 of the cylindrical electrode ... Objective lens magnetic path 13 ... Sample side end 14 of objective lens magnetic path ... Electric field control electrode 15 ... Sample 16 ... Sample stage 17A ... Second detector A for detecting converted electrons
17B ... Second detector B for detecting converted electrons
18A: First detector A for detecting converted electrons
18B: First detector B for detecting converted electrons

Claims (19)

An electron source that generates an electron beam to be a probe;
An aperture for limiting the diameter of the electron beam;
A sample stage on which a sample to be irradiated with the electron beam is mounted;
An electron lens including an objective lens that focuses the electron beam on the sample surface;
When the electron beam passes through the objective lens, decelerating means for decelerating as it approaches the sample,
A deflector for scanning the electron beam on the sample;
A first detector that detects signal electrons that have passed through the objective lens among signal electrons emitted from the sample ; and a second detector;
The first detector and the second detector are disposed between the electron source and the objective lens,
The first sensitive surface that is the sensitive surface of the first detector and the second sensitive surface that is the sensitive surface of the second detector have an axisymmetric shape with respect to the optical axis,
The first detector is arranged to exclusively detect high-energy signal electrons that have passed through the deceleration electric field type energy filter ,
The second detector is placed on the electron source side of the first detector,
Wherein the distance between the first sensing face and the sample side of the distal end portion of the objective lens L1, the distance between the second sensing surface and the sample side of the distal end portion of the objective lens and L2 Then, L1 / a L2 ≦ 5/9, the scanning electron microscope. The first detector, and comprises a signal processing circuit for linearizing adding the output signal from the second detector, according to claim 1 scanning electron microscope according. Wherein the first detector detects backscattered electrons, said second detector for detecting secondary electrons, claim 1 scanning electron microscope according. The retarding field energy filter is wherein the first detector is provided as an independent unit, according to claim 1 scanning electron microscope according. The retarding field energy filter, the first detector and is provided as an integral unit, according to claim 1 scanning electron microscope according.
A deceleration electric field type energy filter is provided on the sample side from the second sensing surface,
The scanning electron microscope according to claim 1 , wherein high-energy electrons that have passed through the decelerating electric field type energy filter are detected separately for each of the first detector and the second detector. The detection solid angle of the first detector facing the tip of the objective lens, the second greater than the detection solid angle of the detector, according to claim 1 scanning electron microscope according. The first detector, and / or, the second detector is a semiconductor detector, an avalanche diode, detector, or a combination thereof using a scintillator material as a micro-channel plate or components, according to claim 1, The scanning electron microscope as described . An electron source that generates an electron beam to be a probe;
An aperture for limiting the diameter of the electron beam;
A sample stage on which a sample to be irradiated with the electron beam is mounted;
An electron lens including an objective lens that focuses the electron beam on the sample surface;
When the electron beam passes through the objective lens, decelerating means for decelerating as it approaches the sample,
A deflector for scanning the electron beam on the sample;
Of the signal electrons emitted from the sample, comprising a first conversion plate that collides with signal electrons that have passed through the objective lens , and a second conversion plate,
The first conversion plate and the second conversion plate are disposed between the electron source and the objective lens,
The first collision surface that is the collision surface of the first conversion plate and the second collision surface that is the collision surface of the second conversion plate have an axisymmetric shape with respect to the optical axis,
The first conversion plate is arranged so that high energy signal electrons that have passed through the deceleration electric field type energy filter collide exclusively ,
It said second conversion plate than said first converter plate is placed on the electron source side,
Wherein the distance between said first impact surface and the sample side of the distal end portion of the objective lens L1, the distance between the second impact surface and the sample side of the distal end portion of the objective lens and L2 Then, L1 / a L2 ≦ 5/9, the scanning electron microscope. The signal electrons impinging on the first conversion plate includes a sensing face for detecting the conversion electrons emitted to the sample side of the impact surface, outside the optical axis, are arranged symmetrically with respect to the said optical axis Comprising first and second detectors;
A sensing surface for detecting conversion electrons emitted from the collision surface to the sample side by signal electrons colliding with the second conversion plate is provided, and the optical axis is arranged symmetrically outside the optical axis. The scanning electron microscope according to claim 9, comprising third and fourth detectors. It said first, second, third, and, the output signal from the fourth detector comprises a signal processing circuit for linear addition, claim 10 scanning electron microscope according. The backscattered electrons to detect the conversion electrons generated collides with the first conversion plate, said second converting plate to the secondary electrons to detect the conversion electrons generated by collision, scanning electron microscope according to claim 9, wherein . The scanning electron microscope according to claim 9, wherein the deceleration electric field type energy filter is provided as a unit independent of the first conversion plate. The scanning electron microscope according to claim 9, wherein the deceleration electric field type energy filter is provided as a unit integrated with the first conversion plate. A deceleration electric field type energy filter is provided on the sample side from the second collision surface,
The scanning electron microscope according to claim 9 , wherein high energy electrons that have passed through the deceleration electric field type energy filter separately collide with each other on the first conversion plate and the second conversion plate. The collision solid angle of the first converting plate facing the tip of the objective lens, the second greater than the crash solid angle of converter plate, according to claim 9, wherein the scanning electron microscope. It said first, second, third, and the detector used in the fourth detector is a detector, or a combination thereof using a scintillator material as a component, according to claim 9, wherein the scanning electron microscope. The scanning electron microscope according to claim 9, wherein the first collision surface and the second collision surface include a material having an atomic number of 50 or more. The scanning electron microscope according to claim 9, wherein the first collision surface and the second collision surface include a material having a negative electron affinity.

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